Interference of Waves: Maximum intensity again

Two 1.0M-Hz radio antennas emitting in-phase are separated by 600m along a north-south line. A radio receiver placed 2.0 km east is equidistant from both the transmitting antennas and picks up a fairly strong signal. How far north should that receiver by moved if it is again to detect a signal nearly as strong.

"Sorry I attached the wrong figure for this question but seems your answer is still right. May I resend the problem with the correct figure and let you to double check with it? If I may, how to attach the correct figure?"